Family D DNA polymerase interacts with GINS to promote CMG-helicase in the archaeal replisome

Crystal structure of the 3'→5' exonuclease from Methanocaldococcus jannaschii

RecJ exonucleases are members of the DHH phosphodiesterase family ancestors of eukaryotic Cdc45, the key component of the CMG (Cdc45-MCM-GINS) complex at the replication fork. They are involved in DNA replication and repair, RNA maturation and Okazaki fragment degradation. Bacterial RecJs resect 5'-end ssDNA. Conversely, archaeal RecJs are more versatile being able to hydrolyse in both directions and acting on ssDNA as well as on RNA. In Methanocaldococcus jannaschii two RecJs were previously characterized: RecJ1 is a 5'→3' DNA exonuclease, MjaRecJ2 works only on 3'-end DNA/RNA with a preference for RNA. Here, I present the crystal structure of MjaRecJ2, solved at a resolution of 2.8 Å, compare it with the other RecJ structures, in particular the 5'→3' TkoGAN and the bidirectional PfuRecJ, and discuss its characteristics in light of the more recent knowledge on RecJs. This work adds new structural data that might improve the knowledge of these class of proteins.

Molecular basis for proofreading by the unique exonuclease domain of Family-D DNA polymerases

Replicative DNA polymerases duplicate entire genomes at high fidelity. This feature is shared among the three domains of life and is facilitated by their dual polymerase and exonuclease activities. Family D replicative DNA polymerases (PolD), found exclusively in Archaea, contain an unusual RNA polymerase-like catalytic core, and a unique Mre11-like proofreading active site. Here, we present cryo-EM structures of PolD trapped in a proofreading mode, revealing an unanticipated correction mechanism that extends the repertoire of protein domains known to be involved in DNA proofreading. Based on our experimental structures, mutants of PolD were designed and their contribution to mismatch bypass and exonuclease kinetics was determined. This study sheds light on the convergent evolution of structurally distinct families of DNA polymerases, and the domain acquisition and exchange mechanism that occurred during the evolution of the replisome in the three domains of life.